The present invention relates to blends of rubber modified monovinylidene aromatic polymers.
Rubber modified monovinylidene aromatic polymers, such as high impact polystyrene (HIPS), have found numerous applications which require high impact strength. One application wherein HIPS is typically used is food packaging products which specifically require good impact strength and transparent properties. Although toughness is achieved by HIPS resins, the transparency of thermoformed articles, such as thermoformed cups, is achieved using an expensive HIPS product having rubber particles with a specific capsule morphology.
Many attempts have been made to acquire the necessary balance of toughness and transparency in thermoformed articles using various materials and techniques. Transparent resins, such as polystyrene, polyethylene terephthalate and polypropylene have been used to produce transparent packaging via in-line thermoforming. However, in-line thermoforming processes require oversize extruders which run slower and are therefore less productive than conventional lines. Additionally, these polymers do not have the impact properties that rubber modified monovinylidene aromatic polymers can achieve. Alternatively, transparent packaging has been prepared by extruding and thermoforming blends of butadiene-styrene copolymer or styrene-butadiene-styrene copolymer with polystyrene. However, the copolymers are expensive and increase cost. Transparent packaging has also been made using optical grade general purpose polystyrene which is produced using tenter frames. However, such products can only be obtained to a maximum sheet thickness of 0.6 mm due to process constraints, which is not thick enough for some thermoforming applications. Polyvinylchloride has also been used to produce transparent packaging, but is thermally unstable and undesirable for food packaging.
Therefore, there remains a need for polymers which can produce cost effective transparent packaging, which can be used in food packaging and non-food packaging markets, and can be produced by conventional thermoforming lines.